This invention relates to an electrophotographic photoreceptor, particularly to one that uses a specified titanyl phthalocyanine pigment as a photoconductive material, that is effectively used with printers, copiers, etc. and that is suitable for image formation with light from semiconductor lasers, LEDs, etc. being used as exposure means. The present invention also relates to a process for producing such photoreceptors.
The use of inorganic photoconductive materials in electrophotographic photoreceptors is increasingly supplanted by organic photoconductive materials. This is because many and various organic photoconductive materials can be synthesized by changing the combinations of starting compounds and process conditions, and this great latitude in the selection of materials permits a desired photoreceptor to be easily fabricated according to a specific object.
The latitude in the selection of materials can be further increased in an electrophotographic photoreceptor of a "functionally separated" type in which carrier generating and transporting capabilities are individually fulfilled by different organic photoconductive materials, and this is expected to provide substantial improvements in electrophotographic characteristics such as chargeability, sensitivity and durability.
There is a growing need of the copying industry for improving image quality and providing copies with an image editing capability. Under these circumstances, efforts are being made to develop a digital copier, printer or other recording apparatuses that meet this need. It is, therefore, strongly desired to improve the performance of photoreceptors used as recording media. In digital recording apparatuses, a photoreceptor is exposed to dots of laser light modulated with an image signal and the resulting latent image dots are developed by a reversal development method. As a laser light source, a semiconductor laser is preferably used since it permits the exposure apparatus to be simplified in design and reduced in size and cost. A semiconductor laser has an oscillation wavelength in the infrared region beyond 750 nm. It is therefore required that the photoreceptor used should have high sensitivity to light in the wavelength range of at least 750-850 nm.
Various organic dyes and pigments have been proposed for use as carrier generation materials in a "functionally separated" photoreceptor and those which are already used commercially include polycyclic quinone pigments typified by dibromoanthanthrone, pyrylium dyes, eutectic complexes of pyrylium dyes and polycarbonates, squaryliun pigments, phthalocyanine pigments, azo pigments, etc. Unexamined Published Japanese Patent Application Nos. 239248/1986, 217050/1986, 67094/1987, 218768/1988, etc. describe titanyl phthalocyanine pigments having predominant sensitivity to light at wavelengths longer than 750 nm. The titanyl phthalocyanine pigments described in these prior patents are designed to have a specific aggregated or crystalline structure so that their predominant absorption is extended into the longer wavelength range to provide a higher sensitivity. However, it is difficult to set optimum conditions for producing the pigments described above and no carrier generation materials have been discovered that are satisfactory in all aspects including chargeability, sensitivity and resistance to cyclic use. In addition, a further improvement in sensitivity is desired.
With a view to meeting this need for higher sensitivity, the assignee previously proposed a photoreceptor using a high-sensitivity titanyl phthalocyanine compound (see Unexamined Published Japanese Patent Application No. 17066/1989 and Japanese Patent Application No. 286537/1988 filed Nov. 11, 1988). This photoreceptor is characterized in that a titanyl phthalocyanine pigment having at least two predominant peaks of the Bragg angle 2.theta. at 27.2.degree..+-.0.2.degree. and 9.6.degree..+-.0.2.degree. in a diffraction spectrum obtained with characteristic X-rays of Cu K.alpha. at a wavelength of 1.54 .ANG. is used as a carrier generation material. The X-ray diffraction spectrum of this titanyl phthalocyanine pigment is entirely different from those of known titanyl phthalocyanine pigments. It has such an aggregated structure that visible and near-infrared absorption spectra show a maximum absorption in the range of 780-860 nm, and hence it exhibits extremely high sensitivity to light from a semiconductor laser.
The titanyl phthalocyanine pigment previously proposed by the assignee has excellent sensitivity characteristics as described above. Further, a photoreceptor using that pigment is suitable for image formation by a digital method, in which said photoreceptor is exposed to dots of laser light modulated with an image signal and the resulting latent image dots are developed by reversal development to produce a toner image in dots. However, the sensitivity characteristics and charge retention of the photoreceptor using said titanyl phthalocyanine pigment are sometimes dependent on the method of dispersing the pigment particles and it is desirable to establish a suitable method of dispersion that is capable of producing a photoreceptor having consistent characteristics.
In ordinary electrophotographic photoreceptors, the electrical contact between a grounded conductive layer and a light-sensitive layer is not microscopically uniform and the efficiency of carrier injection from the conductive layer may differ from one site to another, creating local differences in the distribution of electric charges held on the photoreceptor's surface. These differences will become visible as image defects after development, which are white spots in the black background in a positive-working development process or black spots in the white background in a negative-working reversal development process. In particular, black spots appearing in a reversal development process are as deleterious to image quality as background fogging. This problem is particularly noticeable in photoreceptors provided with high sensitivity by use of the improved titanyl phthalocyanine pigment and the occurrence of black spots is extensive if a reversal development process is adopted.